Single side band modulator



Sept. 25, 1951 L. M. BARTON 2,569,279

SINGLE SIDE BAND MODULATOR Filed Nov. 26, 1949 2 Sheets-Sheet 1 3 PHASE SHIFT NETWORK SHIFT NETWORK SINGLE SIDEBAND OUTPUT 0O PHASE Inventor:

Leo'n M.Bar-ton,

His Attovney- CARRIER INF'UT Sept. 25, 1951 B RT 2,569,279

Fig.2.

8 HHHHHHHI IHHHHHHH MMMAMMMMMMAMMMMMM MMMMW VVV'VV VVVVVVVVVVV Inventor:

Leon MBarton His Att oooo y.

fatenteci Sept. 25, 1951 7 2,5 9,279 I I I I i jsmemsfsinansnn MonuLAToRj Leon Bartom s yracuse, N. .Y., assignor to General Electric New York Company, a corporation of l Application Novemberlzfi, 1949, SerialNo. 129,677 4Claiin's: '(01. 332-45) "I his invention relates" generallyto singleside i band: modulating systems and more particularly to a circuit for effecting single side-band modufj lation of" a carrier'wave by anjaudio signal3*.-

The well known process of amplitude modula tion of acarrier Wave by a modulating signal,

such as is utilized, for instance, in commerc'ial Y radio broadcasting stations, can he expressed mathematically as follows: Esinivt [l+M sin pt]=E sin wt+l Eds the magnitude of the-carrier, wave,

w 18211117111165 the frequency ofthe carrienwav p is 21ptimesthe ,frequency of the modulating signal, 1

M is the modulation .aaagnomny less 131151111 and t representstimein suitable units.

-Theamplitude modulation process represented by-Equation I gives rise to two side-bands, 'sym scrim-supper edema,assesses." Another modulation process can .be performed.

with the same .modulating signal, displaced by 90; electrical degrees, operating ona carrier wave also displaced by,90 electrical degrees from that 5550 ii-1 eds(was (I) used in the firstprocess. Since, the terms sin wt and sin pt :are used to represent. thevoltages involved in the first modulation process. the tenn s cos wt and cos pt representthe same Voltages displaced-by electrical degrees. The m d11-,v lation process maynow be expressed mathewhere the symbols used "have canoe-as m Equation Is the same 1 signifi= ls -plH (wiplftlf This second modulation process alsoproduces two side-band frequencies, s j I 3 t but the aleg braic sign preceding the term representing the upper side-band in this modulation process positive, Whereas it is negative in the first modulation process. The terms repre- 'sentingthe lower side-bands are preceded by similar algebraic signs in both processes. Accordingly, the output of thetwo modulation processes can be added togetherto eliminate the upper side-band, or subtracted to eliminate the lower side-band. Thus adding EquationsI and. IItogether, gives the followings Subtracting Equation II from produces theiollowing The signal represented by Equation III or IV consists only of carrier frequency and the single side -bands respectively. If, instead. of the process of ordinary amplitude modulation which has been described,.a balanced modulation process is utilized, the carrier frequency-iselimin-ated from the out-. put signals expressed by Equations I and 11., Ad!

dition; or subtraction ,then results in the upper,

side band 21s 7 alone, or in the'lower side band alone; respectively.

In the co-pending application #785,259 of Donald E. ,Norgaard, filed November 3, 1941, entitled Selective Side-Band Transmission and Reception. System, and .assignedto the same assignee as the present invention, there is described a system for producing single side-band modulation which utilizesa pair of; tube modulators, I

have found that a pair of bridge crystal modulators may be used for the same purpose with numerous advantages.

Accordingly, it. is anflobject of m invenubnltd provide a new, and improved single side-band generator.

Another object of my invention is to provide a new and improved balanced modulator utilizin bridge circuits in the production of a single sideband output.

For further objects and advantages and for a better understanding of my invention, attention is now directed to the following description and accompanying drawings. The fea-turesof my invention believed to be novel are more particularly pointed out in the appended claims.

In the drawings:

Fig. l is a schematic diagram of a single sideband generator embodying my invention.

Fig. 2 contains a number of curves illustrating the operating characteristics of. the circuit, of

Referring to Fig. 1, an input terminal is adapted to receive a carrier wavesuch-as illustrated by curve 61 of Fig. 2. The carrier wave, is ofra, relatively high frequency with respect to the in tended modulating frequencies. Input terminal I is' -coupled to thecontrol electrodes or grids of a pair oftriode-t-ubes 2" 311(13 which are supp-lied with operating potentialfrom a source indicated by B+L Tubes 2 and 3 operate as cathode coupied amplifiers to provide a suitable impedance conversion-in supplying the carrier wave t the succeedingf stages. Tri'ode tube 3',' in addition to operating as an impedance converter, serves also to introduce a 90 degree phase shift in the car rier wave available a t its output terminah; This is provided by means ofa resistor 4 and an adjustable capacitor 5 coupled, between. its anode andits cathode. Since there is a 180 degree pha d f ereneeb tweea he snal a the anode. and at the cathode, it is'possible, 'by suitable "choice of the resistor 4 and the capacitor 5, to provide a signal, of; any desired intermediate phase. By selecting capacitor 5 so that its recta s t he car er- Wava requ cr s qu to the resistance of resistor G thesignal avail; able at capacitor 1 is advanced in phase by 90 degrees over that available at capacitor 6.

A pair of triode tubes 8 and 9 operate as a paraph amplifi r thecarr er.wavei imn ied. through capacitor 6, and triode tubes l0 and II perform a similarfunction for the carrier wave supplied through capacitor 1. The carrier wave is supplied. to. the control. electrodes. of tubes: & and H1 at 0.-and 9.0 relative phaserespective'ly. Since. tubes 8 and &have. a common cathode impedance constitutedby. resistorl3, the carrier. wave-occurs: at the-control electrode of tube. 9- with @180 degree phase-reversal, in accordance-with prin-' ciples well known in the art. Tubes lll and H operate similarly, and likewise provide a 180 degree phase reversal between the signals at their respective control electrodes. Thus the carrier wave occurs at the anodeslofzthe. tubes Si -and 8.. with respective phases of 0 and 180, and at the anodes of tubes l l and 'lO' with respective phases of 90 and 270.

The signals at the anodes of tubes Sandie-are. coupled through.capacitors Liand' l5.to apair of conjugate. points; A- and B,- of a bridge modulator-- l6 constituted by. four. rectifiers l1, l8, l9; and 20." A pair of adjustable capacitors 2iv and-22, connect points A and-B: toground-respectively. Also resistors 23 and: 214- connect points A and .B respectively, to opposite terminalsofapotentiom eter- 25 whose adjustable tap. connected toground. Capacitors 2| and 22, and also potentiometer 25, are adjusted so that the ca-rrier---wavevoltages at pointsA and- B are of equal amplr tudeand have a-l'BO phasadifierence.

riv d volta s; is maintain d;substan al y. ne,

fier s constituting modulator l6 may consist of diode rectifying, devices such as are well known in the art. In-practice, however, I have found that; germanium, crystal rectifiers serve the pur- F 1 pose verywelLandare more convenient and compact than standard; diode tubes. In addition they have the advantage of much lower capacitance between electrodes and between electrodes and QllIlQi.

A similar circuit comprising bridge modulator sq ,o erates. with. han .T aphas am lifie constitutedbx-tu csln an f inca hacomno ent, parts r n. ever den ical. w h hose bridgemodulator, I 6,1 they will notgbe ,deseribedin, e i B ts F; G andH inm s lat r 0, rrespond. resp c iv ly l'p in s A. B, C, and Din. modulator'lfi. V

The m cl lat ng gn l; esupp iede oan. audio, input terminal 3| which'is, connected. t o -v a 1 8 4 1 of phase shift networks 32 and 33, whi'ch have been designated a and [3 respectively. Thea and 3 phase shift networks introduce a. constant phase difference of 90 between the audio signals at their output terminals. Such phase shiftnetg works are described in the oopending application No. 662,665. of- Robert. B. Dome, entitled fihase Shift-Systemfl-filedi April 17; 1946-; nowpatent- No- 2,566,876 granted septemben 4; 1951; "and assigned to the same assignee as the presentin vention. The a and e phaseshif-t networks have the property of shifting the phase of the two derived voltages at theiroutput'terminals, with--re-'- spect to the. v ltage supplied to h r nput. termial. so hat th P1 2 fis ence, b tw. the

stant at 9Q. asgti e .Voltagei a, wide range of frequencies. In practice, each of the derived output voltages is subject to a phase shift, with respect to the input voltage, whose magnitude is a logarithmic functionof frequencyi.

"However, it is possible to so proportion the circuit constants in both networks that a constant phase difference is maintained between the two derived output voltages. In the present invention.- this. phase. shift. is made..-: degrees; Thefact that there is a total phase shift which varies with frequency: immaterial, and the importantfactor isthattheoutput of the a network-32: has a constant 90 phase difference with respecttotheoutput of the 8.networ.k 33: The-outputvoltage from network-32- may berepresentedby curve 62 of F-ig; 2and the output 'voltagefromnetwork 33 may 'be-represe'nted by a similarcurveshifted" in phase by -90 Theoutputvoltage from net-- work 32 is coupled through capacitor-3fl resistor 35 and radio frequency. choke. 36,.to,-.pQint.D, Qf1 modulator l6. Modulator I6 operates as'a bal anced modulatorsuch thatnovoltage of the carrier wave frequency occurs at point D when the modulat r. is. s itably.. ust d-1 C p citors... and 2l'are used to make h mociulatorv n. a a c ng out. the carrier vqlt:

age. I If the carrier wave voltage is represented by a function of the typesinzgt, and. if the modulatconduction from point A to point D, a low im-" pedance path is effectively constituted between pointsfD and C. On the other 'hand when' the" polarity'of the carrier wave voltage is such that no current passes through the crystal rectifiers; the impedance between 'points D and C is very high. In otherwords', the impedance in one case isiapproximately equal to the positive current'impedance of one crystal which is practically'a' short'circuit, and in the other case, it is approximately equal to the reverse. current impedance of one 'crystal, which is practically an open circuit." The effect of impressing the carrier "w'ave' voltage across points A and B is then "to effectively short circuit'points' D and C atintervalscorresponding to the positive half" cycles of the carrier wave. As' a result, the mod ulatingv voltage which is supplied to' point Dis effectively broken up into a series of pulses having the "frequency of the carrier wave, and the amplitude of the pulses varies in accordance with the'modulating voltage. This is represented by curve .53 of Fig. 2.

The carrier wave frequency is normally much higher than that of the modulating frequency, but, for' purposes of illustration, a lower ratio of carrier to modulating frequency has been shown. I In practice, however, the pulses of voltage'and the carrier wave frequency could hardly be represented to the scale shown in curve 63 of Fig. 21' Curve 63 actually contains a low frequency component equivalent to the modulating frequency, which causes the pulses of voltage to occurabove and below the zero line alternately.

Curve 63 also contains high frequency components corresponding to' the rectangular shape of the pulses of carrier voltage. The low. frequency components corresponding to the modulating frequency are removed by means of coupling capacitor 31 which connects point D to the control electrode of a pentode tube 38, and'preventsinductance 39 from short-circuiting the modulating voltage at point D. The high frequencycornponents'are removed by means of a circuit comprising inductance 39 and a capacitor 40, which are connected in parallel between the control electrode of tube 38 and ground. This circuit is resonant at the frequency of the carrier wave so that higher frequency harmonics are shunted to'ground by capacitor 40. The wave-form of the voltage supplied to the control electrode of tube 38'may be represented b curve 64 of Fig. 2 and this corresponds to a voltage of the form: h I

' cos (w-p)tcos (w-+p)t The first cycle of curve 64 has been drawn to the same frequency scale as shown in curve 63, whereas the remaining cycles are drawn to a frequency scale such as would be used in practice.

The network 33 is connected to point H of modulator 30. A similar modulation process occurs therein, so that the control electrode of pentode tube 39 is supplied with like modulation products resulting from the carrier wave shifted 90. and the modulating wave shifted 90 from those supplied to modulator I6. The signal at the control electrode of tube 39 may be reprethe output voltage at terminal sentedby curve 65 of Fig. 2 andcorresponds to.

asignal of the form: V

' a cos"(w-p)t+'cos (w+p)t (VI) .Thus the signal supplied; to the control elec-e trodes oftubes 38 ,and. 39,. contain both 'sidebands. However, the upper side-band term occurs with a negatiyesign preceding it at, tube 38,,whereas itoccurs with a positive ,sig'n;.pre-, cedingit at tube 39. Both tubes 38 arid 39 have aflcommon, output impedance constituted by an inductance-43, and, a capacitor .44 which form a resonant ..circuit at the lower side bandv frequency. result, the upper side band cancels o ut,- and only the lower side-band 0c,- curs at output terminal 45,. The wave-form of the; lower sideband is,;illustrated by curve .66- of Fig. 2 and may be represented mathematically;

by the function cos; flue-pi 1 Although I have shown a circuit for producing the lower side-band, it. is quite. evident that the circuit may likewise be employed for producing the upper side-band. This may convenientlybe achieved by reversing the connections, from, tubes l0 and ll to terminals E and F of modulator. 30.

Thus by connecting the anodeof tube II] to mod.-

ulator terminal E and the anode of tube II to modulator terminal 15, the upper andlower sidebands in the output of the modulator will-be reversed, so that both will occur with negative signs preceding them in the mathematicaljjequation representative thereof. As a result the.

lower side-bands will cancel outin the }output of tubes38 and 39', and the upper sideiband of the form-cos (w+p t minal 45.-

I have also found that it is possible to achieve abalanced moduation with one pairoflcrystal rectifiers per modulator only.v Thus crystals l1 and IS in modulator "I6 may be replaced byiequal resistances, and likewise for, the correspbnding crystals in modulator .30. f

My invention has the advantage over similar. side-band generators using vacuum tube-modulators, that-a much more accurate balance may be maintained in bridge type modulators such as I have employed. In practical construction of -a single side-band generator embodying my invention, 1 have found that the ratio ofethe rejected side-band and suppressed carrier. to the desired .side-band exceeds 40 decibels; and remains constantover long periods of operation.

In the case of certain vacuumtube modulators, it hasbeen observed-that considerable idrift in the balance point occurred during operation and necessitatedcontinuous readjustment; this difficulty is eliminated by my invention.

While a specific embodiment has been shown and described, it will, of course, be understood that various modifications may be made without ducing from said carrier voltage a derived volt-,

age in quadrature phase, a source of modulating voltage, a pair of phase shift networks for producing two derived modulating voltages having a constant phase difference of a pair of bridge modulators, each of said modulators comrising two pairs of unidirectional conducting will occur at output ter-- 7; deviceshconnected. to;.fo rm parallel conducting paths between arfirst pair of conjugate points, the junctions in the pair of devices in eachpath forming asecond pair of conjugate points, said carrier voltagebeing applied across one pair of said conjugatep'oints of 'one" modulator, and said-carrier voltage of'quadratu're phase being applied to one pair of said conjugate points of said other modulato'rfsaid derived-mod-- ulating voltages being'applied across respective other pairs 'of said-conjugate'points of 'said'mo'dpoints.

72. A single side bandgenerator comprising a source of carrier wave voltage; 'mea'ns for 'producing from said carrier voltage; derived voltages having relative phases of 90,"1'80", ename a- -source of modulating voltage, a pair of'iphaseshift networks for producing, from said modulating voltage, a pair of d'erived modulating voltages having a constant phase "difference of'90, a pair of bridge modulators, each "of said modulators comprising four rectifiers connected "in pairs to form two parallel conducting paths-between one pair of conjugate points, the junctioh of the two alternate conjugate points being connected to-- respective ones of said phase shift networks, and means for algebraically combining the-modulation products at both said other alternate conjugate points.

' 3. A singleside band generator comprising a source of carrier wave voltage, means for producing from said wave derived voltages'of equal ainplitudesl and having relative phase angles 0", 90, 1809 and 270respectively, asourc'e of modulating voltgae, a pair ofphase shiftnetworlis forderiwingfrom'said modulatingvoltage, a pair of modulating voltages having a cOnstantQO' 'phase' difference, a pair of bridge modulators," each of said modulators comprising four' unidire'ctionalconducting devices, said devices being connectedinpairsito form a pair of parallelunidifectional conducting paths between one pair- "of conjugate points, said derived carrier voltagesbeing applied to, said conjugate points, of relativephase 0 and 180 in' voneof said modulators, and of'relative; phase; 190?: and' 270 in. the other of said mo ulators; said derived: modulating voltages being ,two similarly poled,

applied between the junction points, of. saidr dc;-

vices in, said paths-of a respective one ofrsaidg, modulators, whereby modulation products occur;

at'fsaid junctionpoints of eachqmodulator containing upper and lower side-band components sojthat the'components of one side-band areof like polarity at each'of said junction points whereas the components of the opposite side-band are of opposing polarity at 'each of said junction, pointsand a mixing circuit connected to said junction points for algebraically combining saidj modulationside-band components to cancel out thefcemponents forming one side band and retain the components forming theother side band.

"41? A 'sifigl'ej' side" band generator comprising a.

source of carrier wave voltage,'a phase shift net,-

workand apair of'paraphase ampl'ifiers'for'pro,-

ducing,"from said wave, carrier voltages ofedual amplitudes an'd ha ving relative phase anglesfoi 02190, I8l)"and z res'p'ec'tively, a 'sourcbfgn'iodulating v'oltagefa pair of phase shift networks 'for deriving from said modulating voltage, a pairjof modulating voltagesj'having a constant 90 phase" difference, a 'pair'ofbiidge modulators, eachof said modulators comprising four" crysta1 recti fiers, said rectifiers' being paired into two groups and said groups connected in series to, form a pair of parallel unidirectional conducting paths between "one pair of conjugate points, the junction" points of said" rectifiers in said paths forming pairs of alternate conjugate points of'which one is grounded an'dthe other provides a terminal; said produced carrier volta esgof lje1ativepha'se and"l" being applied to respective ones of saidonejpair of conjugate points in one ofj'said' modulators'and said produced carrier ivaves'fof relative" phase" 907' and 270? being applied'to respective ones Of Said: one pair, of conjugate points in said other modulator, said derived modulating voltages being applied to respective terminals of said modulators, whereby modulation products occur at. said 'terminalsfof each modulator containing upper and'low'er side-' band components, so that the componen'tsjof one side-band are of like polarity at each of saidmo'dul'a to'r terminals whereas the components of the opposite side -band are of opposingpolarityfjatj each of 'saidlmodulator terminals, and" a mixing circuit connected tQ' saidterminals for algebraically combining said modulationside band components to cancel out components formingone side band and retain only components forming the other side band.

' LEON M. BARTON.

filiejof this patent UNrrEDsTATEs PATENTS Name Date henehan July' 19, 1949 Number 

